FIELD OF THE INVENTION
The present invention relates to a process for hydrocracking a hydrocarbonaceous feed to make a lubricating oil base stock. In particular, the process of this invention relates to a catalytic hydrocracking process wherein the catalyst system exhibits surprising stability and high viscosity index (VI) selectivity.
The catalyst of the present invention comprises a catalyst having a small amount of zeolite in an amorphous inorganic oxide matrix and containing a hydrogenation component. The catalyst is further characterized as having a significant amount of large pores. In the present process, a hydrocarbonaceous feed is upgraded by reaction over the catalyst system, so that sulfur, nitrogen and aromatic components are removed, and the viscosity index of the lubricating oil base stock is increased relative to that of the feed. The catalyst system also exhibits a high VI selectivity. VI selectivity is a relative measure of the increase in viscosity index during upgrading of a hydrocarbonaceous feed. A high VI selectivity is indicative of a large increase in viscosity index for a given degree of conversion of the feed. The reactions involved in upgrading the hydrocarbonaceous feed according to the present process are generally termed hydrocracking.
Because feeds used in producing lubricating oil base stocks boil up to 1000.degree. F. and above, and contain relatively high nitrogen and sulfur levels, conventional hydrocracking catalysts typically foul quickly. In order to compensate for this high fouling rate, zeolites may be added to the catalysts to increase both activity and stability. However, conventional zeolite-containing hydrocracking catalysts used for upgrading feeds in the preparation of lubes typically have low VI selectivity.
The present invention is based on the discovery of a catalyst containing zeolite and having a pore structure not generally found in lube hydrocracking catalysts which provides both improved stability and improved VI selectivity for the catalyst system.
The pore size distribution of catalysts for hydrotreating heavy oil feedstocks containing metals, particularly residuum feedstocks, have been disclosed in U.S. Pat. Nos. 4,066,574; 4,113,661; and 4,341,625, hereinafter referred to as Tamm '574, Tamm '661, and Tamm '625, and in U.S. Pat. Nos. 5,177,047 and 5,215,955, hereinafter referred to as Threlkel '047 and Threlkel '955. Tamm's patents disclose that heavy oil feedstocks containing metals, particularly residuum feedstocks, are hydrodesulfurized using a catalyst prepared by impregnating Group VIB and Group VIII metals or metal compounds into a support comprising alumina wherein the support has at least 70% of its pore volume in pores having a diameter between 80 and 150 .ANG.. Threlkel '047 teaches that hydrocarbon feedstocks containing metals are hydrodesulfurized using a catalyst prepared by impregnating Group VIB and Group VIII metals or metal compounds into a support comprising alumina wherein the support has at least 70% of its pore volume in pores having a diameter between 70 and 130 .ANG., with less than 5% of the pore volume being in pores having a diameter above 300 .ANG. and less than 2% of the pore volume being in pores having a diameter above 1000 .ANG.. Threlkel '955 teaches that hydrocarbon feedstocks containing metals are hydrodesulfurized using a catalyst prepared by impregnating Group VIB and Group VIII metals or metal compounds into a support comprising alumina wherein the support has at least 70% of its pore volume in pores having a diameter between 110 and 190 .ANG., with less than 5% of the pore volume being in pores having a diameter above 500 .ANG. and less than 2% of the pore volume being in pores having a diameter above 1000 .ANG..
Johnson, in U.S. Pat. No. 5,089,463, discloses a dehydrodemetalation and hydrodesulfurization process using a catalyst comprising a hydrogenation component selected from Group VI and Group VIII metals, and an inorganic oxide refractory support, and wherein the catalyst has 5 to 11 percent of its pore volume in the form of macropores, and a surface area greater than 75 m.sup.2 /g of catalyst.
U.S. Pat. No. 4,699,707 discloses that a full-range boiling shale or fraction thereof is hydrotreated using a catalyst having a surface area in the range of 150 to 175 m.sup.2 /g and a mean pore diameter between 75 and 85 angstroms and a pore size distribution such that at least 75 percent of the pores are in the range of 60 to 100 angstroms.
U.S. Pat. No. 4,695,365 discloses that a spindle oil is hydrotreated using a catalyst having a surface area of at least 100 m.sup.2 /gm and a mean pore diameter between about 75 and 90 angstroms and a pore size distribution wherein at least 70 percent of the pore volume is in pores of diameter in the range from about 20 angstroms below to 20 angstroms above the mean pore diameter.
U.S. Pat. No. 5,171,422 discloses a lube hydrocracking process using a zeolite of the faujasite structure possessing a framework silica:alumina ratio of at least about 50:1.
While these patents generally teach the usefulness of modifying the pore structure of catalysts for treating heavy oils, they do not address the specific problems of achieving high VI selectivity and improved catalyst stability in the hydrocracking of a feed to produce a lubricating oil base stock.